Polypropylene compositions have gained wide commercial acceptance and usage in numerous applications because of the relatively low cost of the polymers and the desirable properties they exhibit. In general, polypropylene polymers, particularly propylene homopolymers, have a disadvantage of being brittle with low impact resistance, especially at low temperatures. To combat these issues, manufacturers have incorporated a dispersed copolymer phase (often called a “rubber” phase), which forms a dispersed phase within the polypropylene matrix. As used herein, the term “rubber” refers to the copolymer of the dispersed phase. These two-phase materials are referred to as impact copolymers or ICPs.
While impact resistance of ICPs is improved, a major drawback to such materials is the poor transparency, mostly due to the rubber particles being large enough to affect light transmission in the heterophasic system. Accordingly, several attempts have been made to improve the transparency of heterophasic polymer systems.
Some ICPs have been able to achieve clarity by blending in metallocene-catalyzed elastomers having the same refractive index as the matrix material. Other ICPs have sought to achieve clarity by increasing dispersed phase miscibility to achieve particles which are not large enough to affect the light transmission. While these approaches may produce clear materials, they generally lack the stiffness and/or toughness of a conventional ICP. It would be desirable to have an ICP which is clear, stiff and tough, and which is not based on either technique.
Other ICPs may lose clarity over time due to a process known as “blooming” where components in the dispersed rubber phase migrates to the surface of the polymeric article over time. Blooming manifests itself as increased haze in the polymeric article or as a whitish coating on the surface of the molded article. This haze can increase over time and the result is aesthetically undesirable and can lead to such issues as decreased shelf appeal in packaged goods.
ICPs can be formed by blending a rubber or dispersed phase with the matrix phase, but it is more preferred, from a cost-to-produce standpoint, that they are prepared using an in-reactor process, where the matrix and the dispersed phase are formed in separate reactors, typically operated in series. Thus, it would be desirable to have an in-reactor formed ICP which is clear, stiff, tough at very low temperatures, resistant to blooming, and which is not based on using rubber with increased miscibility with the matrix.